Hydraulic and other similar fluid control circuits are commonly used in applications wherein a fluid motor is utilized for substantially vertical elevational adjustment of relatively large loads and is at times required to support the load in an elevated position. This type of application results in several problems with respect to the desired function under all conditions as well as safety to the equipment, operator, and service personnel.
For example, most such systems use a sliding spool directional control valve in which the spool must have a radial operating clearance with a bore in the valve body to permit relatively free movement of the spool. When the motor is required to support the load in an elevated position, the annular clearance between the spool and the valve bore provides a leakage path which can result in downward drifting of the load. One solution to this leakage problem is to interpose a poppet type load check valve between the load supporting end of the fluid motor and the control valve to isolate the load supporting pressure from the control valve as disclosed in U.S. Pat. No. 3,127,688 the Hein, et al. When it is desired to lower the load, it is necessary to permit the load check valve to open so that fluid may be displaced from the load supporting end of the motor. This is accomplished in Hein, et al by venting the load check valve across the directional control valve spool when the control spool is actuated to a load lowering position. This has the disadvantages of increasing the overall length of a rather large, expensive control valve and further requires precise machining therein to provide the proper timing relationship of the vent ports in the valve body to the vent grooves in the control spool to insure that the load check is vented only when it is desired to lower the load.
It is also necessary to permit opening of the load check valve when it is necessary to lower the load in the absence of fluid pressure in the system due to a stalled engine or a failure in the control circuit. although the Hein, et al system has this capability with a manually actuated control spool as depicted therein, such system would be operative in a control system utilizing a pilot actuated control spool. In pilot actuated control systems, when pilot pressure fails it is neccessary to bypass the main control spool for returning fluid from the load supporting end of the motor to the reservoir, since pilot pressure will not be available for shifting the control spool to the load lowering position.
Another serious problem attendant to pilot actuated systems, when pressure is available in the pilot control circuit, is inadvertent actuation of the pilot valve to an operative position, which can result in unwanted movement of the load. Such inadvertent actuation of the control system could result in damage to the mechanism being actuated by the fluid motor or serious injury to any persons in the area of the equipment.